The present invention provides for an article which is formed from sheet made by coextrusion of layers of ethylenevinyl alcohol (EVOH), adhesive, polypropylene or other polyolefins such as polypropylene, high density polyethylene, polyethylene and copolymers of polypropylene/polyethylene, or polystyrene and a reground blend of ethylenevinyl alcohol, adhesive, polypropylene and inorganic mineral filler which disperses within the regrind. More particularly, this invention refers to formation of articles such as plastic cups thermoformed from sheets made by coextrusion of the above-mentioned layers of materials in such a way and combination so that delamination of the regrind layers does not occur, and is directed preferably to the unique regrind composition.
Heretofore, extruded polyolefins sheets have been subject to problems of delamination or separation within layers. Delamination has the adverse effects of (a) unsightly defects, (b) loss of part stiffness and (c) loss of impact resistance. Typical, for example, is regrind which contains scrap of polypropylene, ethylenevinyl alcohol and adhesive. Such typical regrind has materials of both high and low viscosity, which tend to separate when subjected to shear forces such as those encountered in a sheeting or film die. The ethylenevinyl alcohol at high shear rates is of higher viscosity than other components and tends to move toward the center of the layer. Such delamination is highly undesirable for articles formed from coextruded sheets of polyolefin layers where increased longevity of the formed articles is desired. In many cases, as with the use of ethylenevinyl alcohol, delamination of the coextruded material layers occurs during forming and/or during flexing while in use before the desired lifetime of the product has expired.
Multilayer sheet containing a layer of oxygen barrier polymer such as EVOH (EVAL.RTM.) as well as layers of adhesive and less expensive polyolefins are being commercially thermoformed into food containers for use in storing shelf stable oxygen sensitive foods. When the parts are formed the unused web of sheet usually represents 30-50% of the sheet fed and must be recovered for reuse. It has been found that this scrap upon regrinding and reextrusion forms a sheet which delaminates too readily upon flexing. This is believed due to a phase separation of the incompatible polar EVOH and non-polar polyolefins in the high shear field inside the sheeting die. This delamination leads to unsightly discontinuities in the formed parts and to loss of stiffness and impact resistance of the parts.
A coextruded material should have an even, uniform output with no surging, have a color suitable to the end use product, and have no recognizably offensive odor. This invention also provides for a method by which the viscosities of the various materials are compatabilized to a similar viscosity range. Some reduction of delamination can be accomplished by use of a dispersing aid, but at the expense of offensive odors and die lip material buildup problems.
The present invention provides for a regrind layer which comprises polypropylene of other polyolefins, ethylenevinyl alcohol, adhesive and an amount of an inorganic mineral filler which will disperse within the regrind to give the coextruded sheet good lamination properties.
In particular, the regrind layer comprises from about 50% by weight to about 95% by weight polypropylene, from about 2% by weight to about 40% by weight ethylenevinyl alcohol, from about 5% by weight to about 40% by weight adhesive and from about 20% by weight to about 60% by weight inorganic mineral filler.
The regrind layer composition is unique in that the inclusion of inorganic mineral filler such as calcium carbonate (CaCO.sub.3) or talc (CaSiO.sub.3) works with the polypropylene, ethylenevinyl alcohol and adhesive to prevent internal delamination in the regrind layer.